Receiving apparatus for train communication systems



l' B52 N52 51 o y A. J. SORENSEN Filed Jan. 2, 1941 ff. I I l 19E 2*Lig/f lgl.

yRECEIVlNG APPARATUS FOR TRAIN COMMUNICATION SYSTEMS Sept. 29, 1942.

Patented Sept. 29, 1942 RECEIVING APPARATUS FOR TRAIN COM- MUNICATIONSYSTEMS Andrew J. Sorensen, Edgewood, Pa., assignor to The Union Switch& Signal Company, Swissvale, Pa., a corporation of PennsylvaniaApplication January 2, 1941, Serial No. 372,902

11 Claims.

My invention relates to receiving apparatus for train communicationsystems.

Railway train communication systems ordinarily use a carrier current.For telephone communication -a carrier current of a preselectedfrequency is modulated by the voice frequency produced 'by vspeaking`into Ya telephone transmitter and a preselected side band of suchmodulated current is transmitted, the carrier and the other side bandbeing suppressed. In Ysuch a system, a calling or signaling 4currentequivalent to the carrier modulated by a preselected frequency withinthe voice frequency range is employed. The receiving apparatus includesa demodulator operable to demodulate and pass only the voice lfrequencyenergy toa telephone receiver or loud speaker in response to receptionof such telephone current. When-calling `or vsignaling current isreceived, energy of the single preselected frequency appears in theVoutput of the demodulator and this 'calling energy is used to govern asuitable signaling or calling device, such as a lamp or electric bell.lI'he loud speaker may be normally set at an inactive condition andvswitched to an active condition upon reception of the calling current.Again the kcalling or signaling current may -be transmitted at all timesexcept when `telephone communication is desired-to continuously checkthe active condition of the apparatus and under such circumstances asignal lamp or indicator is normally energized to indicate to theoperator an active condition of the system, and the loud speaker is heldatan linactive condition as long as such signaling -current is received`and is switched to an active condition when such signaling currentceases.

Accordingly, a feature of my invention is the provision of receivingapparatus for a train communication system incorporating novel andimproved means responsive to a signaling current `signaling currentceases, such second indicator i' being used to inform the operator thattelephone `communication can be expected.

Another feature of my invention is the provi-A sion 'of vreceiving.apparatus for a railway train communication system incorporating asecondary .3.

source of current for vcontrolling the operation of signaling devices.

A further feature of my invention is the provision in receivingapparatus for a train communication system including effective andinexpensive calling and signaling means.

The above cited features of my invention, as Well as other advantageswhich will ybe apparent from the following specification, are attainedaccording to my invention by providing an electron tube, a control relay.and a secondary source of current. The electron -tube is effectivelyiniiuenced by the signaling or calling current to govern the controlrelay which is included in the anode circuit of the tube. The controlrelay in turn governs the secondary source of current which ispreferably a condenser, and the relay and condenser jointly govern theenergization of two different signaling devices. A filter .tuned to passonly energy of the frequency corresponding to that of the signalingcurrent is connected across the output terminals of the usualdemodulator included in such receiving apparatus, and the energy passedby the filter controls the electron tube so that the control relayinterposed in the anode circuit is picked up when such energy is passedby the lter and is released when such energy ceases. The secondarysource of current which is preferably a condenser is maintained chargedas long as the control relay is picked up and is discharged when therelay is released. A rst signal lamp is illuminated when the controlrelay is picked up to indicate reception of signaling current energy. Asignal bell is controlled to sound an audible indication during thedischarge period of the condenser subsequent to the release of thecontrol relay as an indication that signaling current is no longerreceived and telephone communication can be expected. Another or secondsignal lamp is illuminated when the control relay is released as anadditional indication that signaling current is no longer received. Theloud speaker is disconnected from the demodulator as long as the controlrelay is picked up and is automatically connected thereto when thecontrol relay is released.

I shall describe two forms of apparatus embodying my invention and shallthen point out the novel features thereof in claims.

In the accompanying drawing, Figs. 1 and 2 are diagrammatic viewsshowing two different forms of apparatus each of which embodies myinvention. In each of the two views like reference characters are usedto designate similar parts.

Referring to Fig. 1, the reference character IC designates an inductormounted on a vehicle of a railway train in inductive relation to atransmitting circuit, not shown. Ordinarily inductor IC is mounted onthe train in inductive relation to the track rails since the track railsare preferably included in a transmitting circuit to which communicationcurrent is supplied at a station remote from the vehicle on whichinductor IC is located. Such transmitting circuit may be, for example,that covered by Letters Patent of the United States No. 2,112,364,granted March 29, 1938, to L. O. Grondahl, for Electric train signalingsystems.

The electromotive forces induced in inductor IC in response to currentiiowing in the associated transmitting circuit are applied to the inputside of a filter BPF, the output side of which is connected across theinput terminals of an amplifier-demodulator AD. The lter BPF is of theusual band pass type and is proportioned to pass the frequency band ofthe communication current used and to substantially suppress frequenciesboth above and below such frequency band. For example, it has beenproposed for communication systems for railway trains to employ theupper side band of a carrier current of 6,000 cycles per secondmodulated by voice frequency for telephone communication and to use theequivalent of such carrier current modulated by a single preselectedfrequency within the voice frequency range for calling and signaling.Under such circumstances, the lter BPF would pass a band extending from6,500 to 8,500 cycles, the voice frequency range being taken asextending from 500 cycles to 2,500 cycles. For signaling and calling thecurrent transmitted may be of 7,000 cycles which is the equivalent ofthe upper side band of the carrier of 6,000 cycles modulated by thevoice frequency of 1,000 cycles.

The amplifier-dernodulator AD may be any one of several well-known typesfor such devices and is shown conventionally for the sake of simplicitysince the specific structure of the amplifier-demodulator forms no partof my present invention. It is suiiicient for this application to pointout that the amplifier-demodulator AD includes a normally active sourceof carrier current for mixing with the incoming communication current,and is operative to cause only the voice frequencies to appear at theoutput terminals of the demodulator, the local carrier and the otherproducts of demodulation being suppressed. One or more stages ofamplification may be employed in the amplifier-demodulator. I'

On the assumption that the communication current is of the frequencyrange referred to above, voice frequencies extending from 500 to 2,500cycles appear at the output terminals of the amplier-demodulator whentelephone current is picked up by the inductor IC and energy of thesingle preselected frequency of 1,000 cycles appears at the outputterminals of the demodulator when signaling current is picked up byinductor IC. It will be understood of course that my invention is notlimited to the frequencies set forth hereinbefore but such are used forillustration only.

A filter FI including a reactor 2, a transformer TI and a condenser 3,is connected across the F output terminals of the ampliiier-demodulatorAD, the arrangement being that primary winding 4 of transformer TI,condenser 3 and reactor 2 are connected in series across the outputterminals of the amplifier-demodulator. The parts of filter FI areproportioned so as to pass only energy of the frequency of the signalingcurrent, that is, to pass only energy of 1,000 cycles per second whensignaling current of the frequency referred to hereinbefore is used.

The apparatus includes an electron tube ET, a source of alternatingcurrent CS, a control relay RI, and two condensers CI and C2. Theelectron tube ET is preferably of the cold cathode gas filled type, andit is provided with an anode 'I, a control element 8 and a cathode 9.The source of alternating current may be any suitable means and is shownas comprising a vibrator VB and a transformer T2. An operating windingI0 of vibrator VB is connected over its own armature II and contact I1to a suitable source of direct current, such as the usual train carried32 volt generator or battery, not shown, but whose terminals areindicated at B32 and N32. Armature II of vibrator VB is caused tovibrate as long as operating winding I0 is energized, the rate ofvibration being predetermined by the proportion of the parts. ArmatureII when vibrated alternately engages contacts I2 and I3 and completes acircuit by which current from the train carried source is alternatelysupplied to two portions of primary winding I4 of transformer T2 withthe result that an alternating electromotive force is induced insecondary winding I5 of transformer T2. The parts are so proportionedthat the electromotive force induced in secondary winding I5 is of avoltage of say 110 volts and of a frequency of say 60 cycles per second.It will be understood of course that other voltages for the alternatingcurrent source and other frequencies for the alternating currentsupplied may be used if desired. A condenser I6 is connected acrosscontact II of vibrator VB to suppress sparking and a condenser I8 isconnected across secondary winding I5 of transformer T2 to improve theoperation. Tube ET is provided with an anode circuit which includes thelower terminal of secondary winding I5 of transformer T2, anode 'I andintervening tube space to cathode 9 of tube ET, a resistor 2l andwinding I9 of control relay RI to the upper terminal of the secondarywinding I5, condenser CI being preferably connected across the windingI9. Relay RI is a direct current relay of the usual acting type. Acontrol circuit for tube ET includes a secondary winding 6 oftransformer TI connected across control element 8 and cathode 9 of tubeET. When no electromotive force is applied to control element 8 of tubeET the voltage applied across anode 1 and cathode 9 by the source ofalternating current CS fails to ionize the tube and no current flows sothat control relay RI is released. Under this condition condenser C2,which is a relatively large condenser, is connected across winding I9 ofrelay RI over back contacts 22 and 23 of a relay R2 to be laterdescribed, such connection being obvious from an inspection of Fig. l.When energy is passed by filter FI due to reception of signalingcurrent, the control element 8 of tube ET is driven more positive inpotential with respect to cathode 9 during the positive half cycle ofsuch energy and tube ET is ionized so that current flows in the anodecircuit. Since the signaling energy is of the frequency of 1000 cyclesper second and each positive half cycle drives control element 8 morepositive in potential, tube ET is ionized and rectified current flows inthe anode circuit as long as the signaling energy is present. Due to theflow of such rectified current in the anode circuit, condenser C2.gradually accumulates a charge and 'a voltage .gradually builds up atthe -tvvo terminals fof the condenser and therefore builds 'up acrossWinding |19 of relay Rl. When the voltage reaches the pick-up value ofrelay RI, the relay isV pickedup. When relay RI is picked up Vto closefr'ont contact 24, a circuit easily traced is completed by Which relayR2 is connected across terminals B32 and N32 of the train carriedcurrent source and relay R2 is picked up to open back `contacts 22 and23 to disconnect condenser C2 from across winding I9 of relay RI.Condenser Cl which is also connected across winding 19 may be relativelysmall in capacity and serves to prevent the current through winding i9from falling o'if too muc-h when condenser C2 'is disconnected. Whenrelay R2 is picked up to close front contacts 25 and 25, the condenserC2 is connected '-to `the train carried current source vover a circuitextending from terminal B32, lfront contact 24 of relay Rl, frontcontact 26 of relay R2, condenser C2, front contact 25 of relay R2, andto terminal N32, and condenser C2 is maintained charged as long as relayRl is picked up. That is to say, condenser C2 is maintained charged aslong as signaling current is received. When relay R2 is picked up asignal lamp L'I is illuminated over an obvious circuit including frontcontact y2'! of relay R2. over an easily traced circuit including lback-contact 28 of relay R2. Another signaling device 'in the form of anelectric bell BI is provided and Iis energized over a circuit extendingfrom terminal B32, front contact 21 of relay R2, back Contact 29 ofrelay Rl, operating Winding of bell BI and to terminal N32.

A loud speaker LS is included in the apparatus and as here shown isprovided With van energizing Winding 30 preferably connected ytoterminals B32 and N32 of the current source and with an operatingWinding 3l connected across the output terminals of the amplier-demodulator AD over an obvious circuit including back contact 32 of relay R2.

Thus, as long as signaling current Vis received and energy is passed bylter FI, relays Rl and R2 are picked up, condenser C2 'is maintainedcharged, lamp LI is illuminated to indicate the presence of suchsignaling current and the apf paratus is in a werking condition, and theloud speaker LS is maintained in an inactive condition.

Assuming such signaling current is no longer transmitted then thealternating current applied to the anode circuit of tube ET causes thetube to be deionized during the negative half cycle of the current andrelay RI is released. With 4relay Rl released to open front contact 24,the 4circuit for relay R2 and the circuit by which condenser C2 ismaintained charged are open. Condenser C2 now discharges through theWinding of relay R2, since it is connected across that Winding overfront contacts 25 and 26 of the relay. Relay R2 is therefore retainedenergized and picked up during the discharge interval of condenser C2.During the interval relay Rl is released closing back contact 29 andrelay R2 is picked up closing front contact 21, the bell B'I isenergized to provide an audible indication 'that reception of signalingcurrent has ceased and that telephone communication can be expected.YWhen condenser C2 is discharged relay R2 is deenergized and releasedand lamp LI is extinguished and lamp L2 is illuminated. The release ofrelay R2 A second signal lamp L2 is controlledy #to close back contact32 also connects ,the loud speaker to Sthe ampliner-demodulator so thatwhen vtelephone current is subsequently 'received `the voice frequenciesare reproduced by the :loud speaker.

Itis vtc be .pointed out 'that When the telephone current lcontains thevoice frequency of 1,000 cycles, energy is passed by filter FI and tubeET may be ionized so that current flows in the anode circuit Vof thetube. Relay .Rl is not however immediately picked up because condenserC2 must rst :be charged and a voltage build up across its terminalsequal to the pick-up value of relay R-l, and the 'interval thus requiredto charge condenser C2 lis greater than that during Which the voicefrequency of 1,000 cycles ordinarily exists. When `however the telephonecurrent ceases and signaling current is :again transmitted so lthatenergy is passed by filter FI to cause tube ET to'fbe ionizedy thenrelay RI is 'picked up subsequent to the 'charging vof condenser C2 andthe 'apparatus 'is restored to th'e normal position, that is, to theposition shown in Fig. 1.

In Fig. 2, the apparatus of -Fig. 1 fis modified by relay R2 beingomitted, `condenser C2 being charged from the .train carried source ofdirect current, by the energy 'stored in condenser C2 being used "tooperate bell BI `directly and by vcondenser Cl Vbeing replaced by arelatively large condenser C3.

It is believed that 'the apparatus of Fig. l2 Will be best understoodfrom a description of its operation. When signaling current Vis firstreceived, 'the energy passed by filter FI causes tube ET to ionize andcurrent to iiow in the anode circuit. Condenser C3 being of relativelylarge capacity gradually 'builds up la charge and after a predeterminedinterval the voltage across its terminals reaches the pick-up value ofcontrol relay Rl and the relay is picked up after which it is heldpicked up as long as signaling current is received. When relay RI ispicked up to close front contact 33, condenser C2 is connected acrossterminals B32 and N32 and is charged and vis maintained charged as longas relay RI is held picked up. The closing of front contact 34 of relayRl completes a simple circuit 'for lamp Ll and that lamp is illuminatedto indicate reception of signaling current. The operating Winding 3i ofloud speaker LS is disconnected from 'the amplier-demodulator AD at backcontact 35 of relay Rl.

When the signaling current ceases, the alternating current supplied tothe anode circuit of tube VET causes lthe tube to be deionized and relayRI to be released. With relay RI released to open front contact 33 andto close back contacts 33 and 31, condenser C2 is disconnected from thecurrent source 'and is connected tobell BI. The energy stored incondenser C2 is now discharged through the operating winding of bell BI,and bell BI is sounded during the discharge period of condenser C2 toprovide an audible indication 'that signaling current is no longerreceived and that telephone communication can be expected. With relay RIreleased to open front `contact 34, lamp LI is extinguished and with theclosing of back contact 38 a simple circuit is formed for lamp L2 andthat lamp is illuminated to indicate that signaling current is no longerreceived4 The closingof 'back Contact 35 of relay Rl eiectively connectsvloud speaker LS to the output terminals of the ampliiier-demodulator ADand hence when telephone vcurrent is received the voice frequencies -arereproduced at the loud speaker. It is to be pointed out that When thetelephone current contains the voice frequency of 1,000 cycles andenergy is passed by lter Fl so that tube ET is ionized, relay RI is notpicked up until after condenser C3 is charged with the result that relayRI will not ordinarily be picked up under such circumstances because ofthe short duration of such frequency of the telephone 'current.

It is to be seen, therefore, that I have provided novel and inexpensiveselective means for train communication systems Wherewith a rst signallamp is illuminated to indicate reception of signaling current, a signalbell is sounded during a predetermined interval as an indication thatsignaling current is no longer received and that telephone communicationcan be expected, a second signaling lamp is illuminated to indicate theabsence of such signaling current, and the loud speaker is maintained inan inactive condition as long as signaling current is received and isautomatically switched to an active condition when such signalingcurrent ceases.

Although I have herein shown and described only two forms of receivingapparatus for train communication systems embodying my invention, it isunderstood that various changes and modifications may be made thereinWithin the scope of the appended claims Without departing from thespirit and scope of my invention.

Having thus described my invention, what I claim is:

l. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency Within said voice frequencyband, an electron tube, a control relay, means to connect said electrontube to said filter and said relay to said tube to cause energization ofsaid relay as long as energy is passed by the filter, a condenser, meanscontrolled by said relay to maintain said condenser charged as long assaid relay is energized, and means energized by the discharge of saidcondenser when said relay is deenergized to provide an indication.

2. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a lterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency Within said voice frequencyband, an electron tube, a control relay, means to connect said electrontube to said filter and said relay to said tube tocause said relay to beenergized and picked up as long as energy is passed by said filter, acondenser, means including a front contact of said relay to maintainsaid condenser charged, and means controlled by the discharge of saidcondenser when said relay is released due to energy being no longerpassed by said filter, to provide an indication for a predeterminedinterval.

3. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected f frequency Within said voicefrequency band; a gas filled electron tube having an anode, a cathodeand a control element, a control relay, a source of alternating current,an anode circuit including a Winding of said control relay and saidsource of alternating current connected to the anode and cathode of thetube to energize saidrelay when said tube is ionized, a control circuitto connect said filter across the control element and cathode of thetube to cause ionization of the tube by the energy passed by the filter,a condenser, means including a contact closed when said relay isenergized to maintain said condenser charged, and means controlled bythe discharge of said condenser when said relay is released due toenergy being no longer passed by said filter to provide an indicationduring the discharge period of the condenser.

4. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency Within said voice frequencyband, an electron tube, a control relay, means to connect said electrontube to said filter and said relay to said tube to cause said relay tobe energized and picked up as long as energy is passed by said filter,another relay, a condenser, a circuit including a front contact of saidcontrol relay to energize and pick up said other relay, means includinga front contact of said other relay in series with a front contact ofsaid control relay to maintain said condenser charged, means including afront contact of said other relay to connect said condenser to saidother relay to maintain said other relay energized by the discharge ofthe condenser subsequent to release of said control relay, and asignaling device energized over a circuit including a back contact ofthe control relay in series with a front contact of said other relay toprovide during the discharge period of said condenser an indication thatenergy is no longer being passed by said lter.

5. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency Within said voice frequencyband; a gas filled electron tube having an anode, a cathode and acontrol element; a control relay, a source of alternating current, ananode circuit including a winding of said control relay and said sourceof alternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said lter across the control element and cathode of the tube tocause ionization of the tube by the energy passed by the filter, acondenser, another relay, means including a back contact of said otherrelay to connect said condenser across the Winding of said control relayto delay the picking up of the control relay when the tube is ionized,means including a front contact of said control relay to energize saidother relay, and a signal lamp energized over a front contact of saidother relay.

6. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a. given voice frequency band, a lterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency Within said voice frequencyband; a gas filled electron tube having an anode, a cathode and acontrol element; a control relay, a source of alternating current, ananode circuit including a Winding of said control relay and said sourceof alternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said filter across the control element and cathode of the tubeto cause ionization of the tube by the energy passed by the lter, acondenser, another relay, means including a back contact of .said otherrelay to connect said condenser across the Winding of said control relayto delay the lpicking up of the control relay until said condenser is:charged When the tube is ionized, means including a front contact ofsaid control relay to pick up said other relay, means including a frontcontact of said control relay and of said other relay to maintain saidcondenser charged, means including a front contact of said other relayto connect the condenser across the winding of said other relay to delayits release due to the discharge of the condenser when said controlrelay is released, a first signal device controlled over a front contactof said other relay and a second signal device controlled over a backcontact of the control relay in series with a front contact of saidother relay.

7. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current 4consistingof a carrier current modulated -fby a `given voice frequency band, afilter connected across the output terminals of said demodulator andtuned to pass only energy of a preselected frequency Within said voicefrequency band; a gas lled electron tube having an anode, a cathode anda control element; a control relay, a source of alternating current, ananode circuit including a Winding of said control relay and said sourceof alternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said lter across the control element and cathode of the tube tocause ionization of the tube @by the energy passed by the filter, acondenser, another relay, means including back contacts of said otherrelay to connect said condenser across the Winding of said control relayto charge the condenser yvhen the tube is ionized and delay for apredetermined period the picking up of the control relay, meansincluding a front contact of the control relay to pick up said otherrelay, means including a front contact of the control relay and a frontcontact of the other relay to connect said condenser to a direct currentsource to maintain the condenser charged, means including the lastmentioned front contact of the other relay to connect said condenseracross the Winding of said other relay to provide a preselected slowrelease period for said other relay when the control relay is released,a signal lamp illuminated over a front contact of said other relay, anda signal bell energized over a back contact of said control relay inseries With a front contact of said other relay. t

8. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a ,communication current consistingof a carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a, preselected frequency within said voice frequencyband; a gas filled. electron tube having an anode, a cathode and acontrol element; a control relay, a source of alternating current, ananode circuit including a Winding of said control relay and said sourceof alternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said filter across the. control. element and cathode of the tubeto cause ionization of the tube by the energy passed by the filter, acondenser, another relay, means including back contacts of said otherrelay to connect said condenser across the Winding of said control relayto charge the condenser VWhen the tube is ionized and delay .for apredetermined period the picking up of the control relay, meansincluding a front contact of the control relay to pick up said otherrelay, means including a front contact of the control relay and a frontcontact of the other relay to connect said condenser to a direct currentsource to maintain the condenser charged, means including the lastmentioned front contact of the other relay to connect said condenseracross the Winding of said other relay to provide a preselected slovvrelease period for said other relay when the control relay is released,a first signal lamp controlled over a front Contact of said other relayto provide a continuous indication as long as energy is passed by saidfilter, a signal bell energized over a back contact of said controlrelay in series with a front contact of said other relay to provide anaudible indication for a preselected interval when energy ceases to bepassed by said lter, `and a second signal lamp controlled over a backcontact of said other relay to .provide a continuous indication whenenergy is not being passed by the filter.

9. Receiving apparatus for a train communication system comprising, ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a lterconnected across the output terminals of said demodulator and tuned topass only energy of a prese- 'lected frequency Within said voicefrequency band; a gas lled electron tube having an anode, a cathode anda control element; a control relay, a source of alternating current, ananode circuit including a Winding of said control relay and said sourceof alternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said lter across the control element and cathode of the tube tocause ionization of the tube by the energy passed by the lter, acondenser, a source of direct current, means including a front contactof said control relay to connect said condenser to said direct currentsource to charge the condenser, a first signaling device controlled overa front contact of said control relay to indicate when energy is beingpassed by said lter, a second signaling device, and means including aback contact of said control relay to connect said condenser to saidsecond signaling device to energize that signaling device by thedischarge of the condenser to indicate When energy is no longer beingpassed by the filter.

10. Receiving apparatus for a train communication system comprising. ademodulator operable to demodulate a communication current consisting ofa carrier current modulated by a given voice frequency band, a filterconnected across the output terminals of said demodulator and tuned topass only energy of a preselected frequency within said voice frequencyband; a gas lled electron tube having an anode, a cathode and a controlelement; a control relay, a source of alternating current, an anodecircuit including a winding of said control relay and said source ofalternating current connected to the anode and cathode of the tube toenergize said relay when said tube is ionized, a control circuit toconnect said filter across the control element and cathode of the tubeto cause ionization of the tube by the energy passed by the lter, afirst condenser connected in multiple With the winding of the controlrelay to delay the picking up of the control relay until said condenseris charged to require that energy be passed by said filter for apredetermined period to pick up the control relay, a second condenser, acircuit including a front Contact of said control relay to connect saidsecond condenser to a source of direct current to charge that condenser,a signal lamp controlled over a front contact of the control relay toindicate When energy is being passed by said filter, an electric signalbell, and a circuit including a back contact of said control relay toconnect said second condenser to said bell to energize the bell by thedischarge of the condenser to indicate when energy is no longer passedby the filter.

11. Receiving apparatus for a train communication system using a carriertelephone current consisting of a carrier current modulated by a givenvoice frequency band and a signaling current equivalent to said carriercurrent modulated by a single preselected frequency within said voicefrequency band, comprising, a demodulator operable to demodulate suchtelephone and signaling current when such currents are applied thereto,a filter connected to the output terminals of said demodulator and tunedto pass only energy of said preselected frequency, a gas filled electrontube, a control relay, a source of alternating current, an anode circuitincluding said source of alternating current and control relay connectedacross the anode and cathode of said tube to energize the relay when thetube is ionized, a control circuit to connect the output side of saidfilter across the control element and cathode of said tube to causeionization of the tube by the single frequency energy passed by thefilter, a condenser, means including a front contact of said controlrelay to connect said condenser to a source of direct current to chargethe condenser, a signal lamp controlled over a front contact of saidrelay to indicate when said signaling current is applied to saiddemodulator, an electric bell, means including a back contact of saidrelay to connect said condenser to said bell to energize the bell by thedischarge of the condenser when such signaling current ceases to beapplied to the demodulator, a loud speaker, and means including a backcontact of said relay to connect an operating Winding of the loudspeaker to the output terminal of the demodulator to reproduce the voicefrequencies when said telephone current is applied to the demodulator.

ANDREW J. SORENSEN.

